Ink jet printing is a non-impact method for producing images by the deposition of ink droplets in a pixel-by-pixel manner to an image-recording element in response to digital signals. There are various methods that may be utilized to control the deposition of ink droplets on the image-recording element to yield the desired image. In one process, known as continuous ink jet, a continuous stream of droplets is charged and deflected in an imagewise manner onto the surface of the image-recording element, while unimaged droplets are caught and returned to an ink sump. In another process, known as drop-on-demand ink jet, individual ink droplets are projected as needed onto the image-recording element to form the desired image. Common methods of controlling the projection of ink droplets in drop-on-demand printing include piezoelectric transducers and thermal bubble formation. Ink jet printers have found broad applications across markets ranging from industrial labeling to short run printing to desktop document and pictorial imaging.
The inks used in the various ink jet printers can be classified as either dye-based or pigment-based. A dye is a colorant that is dissolved in the carrier medium. A pigment is a colorant that is insoluble in the carrier medium, but is dispersed or suspended in the form of small particles, often stabilized against flocculation and settling by the use of dispersing agents. The carrier medium can be a liquid or a solid at room temperature in both cases. Commonly used carrier recording materials include water, mixtures of water and organic co-solvents and high boiling organic solvents, such as hydrocarbons, esters, ketones, etc.
For aqueous dye-based inks, the dyes needs to be sufficiently soluble in water to prepare a solution that is capable of producing adequate density on the receiving element and stable for extended periods of storage without precipitation. High quality ink jet printing with dye-based inks requires dyes that will provide both bright hue and good light fastness.
To generate full color prints via ink jet printing, ink sets comprising at least cyan, magenta and yellow inks are normally utilized. In addition a black ink is often added to enhance the printing of text and darker colors. The range of colors that can be produced with a given set of inks defines the color gamut of that ink set. For the production of high quality photorealistic images via ink jet printing, ink sets with a large color gamut are preferred. In addition, it is important that the ink sets produce images with good fastness, especially to light.
The choice of the colorants in ink jet systems is critical for both light fastness and color gamut. The color gamut of an ink set is controlled primarily by the spectral absorption characteristics of the component dyes. The primary dyes (e.g., cyan, magenta and yellow) should only absorb light of the required wavelengths (i.e., have relatively narrow absorption bands) and not overlap excessively with the dyes in the complementary inks.
Of particular importance is the light fastness of the component dyes when printed on recording materials that are subsequently sealed with a laminating film. Lamination is done to protect the printed image from physical damage and also to further enhance the stability of the printed image against light fade and fade due to environmental pollutants such as ozone.
Recording material laminate films are available in a variety of forms and can be categorized based on their mode of adhesion to the printed recording material. These forms include heat activated laminates, thermal laminates and pressure sensitive laminates. Laminate films can contain ultraviolet light absorbing compounds that can in part protect the printed dyes from fade due to ultraviolet light.
Numerous dye based ink jet ink sets have been described in patent literature with varying performance in terms of color gamut and light fastness on laminated media. In order to achieve a high color gamut, U.S. Pat. No. 5,679,140 describes preferred spectral characteristics for dyes used in ink jet ink sets. In particular, specifications are given for the preferred spectral characteristics of yellow dyes. However, the patent indicates no preference for selection of black dyes and does not address the need for good laminated light fastness for the set of dyes in the ink jet ink set.
Water soluble, metal complex black dyes are known in the art to have good light fastness and are described in for example U.S. Pat. No. 5,725,641, U.S. Pat. No. 6,302,949 B1 and U.S. patent application Publication No. 2001/0027734A1. The latter patent application discloses use of a trisazo metal complex black dye in an ink jet ink set with cyan, magenta and yellow dyes. However, the yellow dyes cited in 2001/0027734A1 have neither the preferred spectral characteristics to achieve a high color gamut nor high light fastness.
U.S. Pat. No. 6,468,338 B1 describes water soluble pyrazoleazoindole yellow dyes with good light fastness although no preference is indicated for the other dyes in an ink jet ink set. Likewise, U.S. patent application Publication Ser. No. 10/732,093, filed Dec. 10, 2003, describes water soluble azo pyrazole-azole yellow dyes with good light fastness without describing other preferred dyes to complete an ink jet ink set.
There is still needed an ink set capable of producing images with excellent color and also with high light fastness when printed on laminated recording materials.